350 



THE AGRICULTURAL NEWS. 



October 26, 1912. 



FUNGUS NOTES. 



A KNOT OF CITRUS TREES. 



In Bulletin No. 'J47 of the Bureau of Plant Industry 

 of the United States Department of Agriculture, entitled 

 A Knot of Citrus Trees Caused by Sphaeropsis Tumefaciens, 

 by Florence Hedges and L. S. Tenny, an account is given of 

 a peculiar knot disease of limes and oranges in Jamaica. 

 The causative organism is described at some length, with its 

 cultural characters, and details of numerous inoculation 

 experiments, almost all successful, are also provided. The 

 same or a very similar disease has also been recorded 

 in Florida but it has never been observed up to 

 the present in any of the smaller West Indian Islands. 

 A few years ago the damage caused by this disease in 

 Jamaica was of some importance on certain estates, but 

 recently by severe pruning, and by burning badly diseased 

 trees, it has been brought under effective control. 



SYMPTOMS. The knots may appear on the stem or 

 branches, and vary in size from |-inch to 2 or 3 inches in 

 diameter. 'Generally they are approximately round, but in 

 the earlier stages before they surround the stem they have 

 a broad, flattened base, and may be somewhat elongated, the 

 longer axis being parallel to that of the infected branch. 

 In some cases, instead of forming a well-defined gall, the 

 disease produces an eruption extending several inches along 

 the limb. This consists of slightly raised and variously 

 fissured portions. 



'At first the incipient knot is covered by the bark, but 

 after some months this usually dies and falls away, exposing 

 the enlarged woody tissues. In the early stages of growth 

 the surface of the knot (the bark) is light coloured and 

 rather smooth. With age the surface becomes darker and 

 more or less fissured, and an old gall divested of its bark is 

 often almost black, rough, knotty, and deeply furrowed. 

 When the growth is rapid, however, the malformation may 

 reach a large size and the bark surface still be intact, 

 smooth, and light in colour. 



'The surface of the knot is usually quite different in 

 texture from that of the healthy stem; the firm greenish 

 bark is replaced by a softer, more or less crumbling surface, 

 cutting easily with a knife. The interior of the knot, 

 however, is very hard and compact, being composed of firm 

 woody tissues. The outer layer of the knot (modified bark) 

 is about the thickness of ordinary bark, i.e , the knot grows 

 chiefly by additions to the woody, not to the cortical tissues. 

 The colour of the interior of the knot may be similar to that 

 of healtTiy wood, or it may be more or less streaked with 

 black. The knots are attached by a broad base and cannot 

 be broken off. As they grow they extend laterally and may 

 ultimately girdle the branch. 



'The first indication of infection is a slight swelling of 

 the branch. As this increases in size the bark, which at this 

 stage covers the young knot, becomes lighter in colour and is 

 noticeably cracked and has a cork like appearance. Growth 

 under greenhouse conditions in Washington is slow, several 

 to many months being necessary for the production of knots 

 1 to 3 inches in diameter. When many knots appear upon 



a branch the single ones may be smaller. In the artificial 

 infections in the Department greenhouses it has taken from 

 five months to two years for the knots to girdle the branch 

 and to become 1 to 2 inches in diameter, but our temperatures 

 were considerably lower and the moisture less than in 

 Jamaica. By the time the stem is girdled all that part of 

 the branch above the knot is usually dead or dying. When 

 a tree contains numerous infections, therefore, a large portion 

 of it dies.' 



These knots often give rise to a large number of 

 branches, from two to forty or more, and thus form typical 

 witches' brooms. The branches may attain a height of 

 several feet with few if any branchlets, and often themselves 

 bear knots. They are of short duration and die after some 

 months. Gumming also occurs from the knots, and gum 

 pockets are of frequent occurrence in the infected wood. 



Since, when a knot encircles a branch, the upper portion 

 of the branch dies, a badly infected tree may have so much 

 dead wood as to be commercially useless. Moreover, the 

 causative fungus can spread in the stems and give rise to 

 secondary knots, which, on neglected trees, arise in large 

 numbers on the branches and main stem and may extend to 

 the ground, so that the tree ultimately dies. 



The disease attacks trees of all ages and appears on old 

 as well as young wood; its occurrence is independent of 

 weather conditions, but it was noted in Jamaica to be more 

 prevalent on the so-called 'red dirt', which is rich in iron. 

 In that island affected trees are usually killed in about 

 eighteen months; cutting back the affected tree did not appear 

 to be always successful, as the young shoots springing from 

 below developed knots. 



THE CAUSATIVE FUNGUS. The disease is due to a fungus 

 named by the investigators who isolated it Sp/iaeropsis 

 tumefaciens. It possesses a mycelium that is white at first, 

 but becomes brown when older by the colouration of the cell- 

 walls of the hyphae. Many of its cultural characters are 

 similar to those of Thyridaria tarda and of Dijnodia 

 natalensis, but it is a species quite distinct from these, as is 

 shown by the shape and colour of its pycnidiospores. The 

 mycelium occurs in the bark and wood of the knots, and in 

 the bark, wood and pith of the stem. In rapidly growing 

 galls the mycelium is not very plentiful and is often colourless, 

 while in those developing more slowly it may occur in quan- 

 tity sufficient to cause a blackening of the tissues. Three 

 distinct strains were isolated by Miss Hedges and Tenny, two 

 sterile on culture media, one from limes, and one from 

 oranges, and a vigorously fruiting strain from oranges. The 

 fructifications are of two kinds, pycnidia and so-called sper- 

 magonia. These are also formed very freely in some cases 

 on knots on inoculated or diseased plants. The pycnidia on 

 the host are produced beneath the epidermi'i. hut eventually 

 burst through it. They are small, round, l.laik and usually 

 crowded together. They contain colourless or yellow spores 

 oblong or ovoid, usually unicellular, sometimes 1-, 2- or even 

 4septate, especially in cultures. The spermagonia are some- 

 times the actual pycnidia, sometimes are formed separately 

 and occur after the pycnidia; they produce small unicellular 

 colourless spore-like bodies, which, however, have not been 

 observed to germinate. 



INOCULATION EXPEEiJiENTS Almost all the inoculations 

 were made by inserting spores or mycelium of the fungus, or 

 both, into cuts in the stems of the host plants, so that the 

 majority were wound infections. An attempt to infect an 

 old healed wounded surface, without further damaging it, 

 failed. The summary of the re-i^ults of the inoculations as 

 given in the Bulletin under consideration is as follows: — 



